The present invention relates to a control system for selecting a two wheel drive mode or a four wheel drive mode in a so-called part-time four wheel drive vehicle.
A conventional system of this kind is disclosed in "Motor Fan" (Japanese magazine), October 1984, page 101. In this system, a 2WD-4WD change-over unit is connected with a brake switch for turning on stop lamps when the brake is applied. The change-over unit is normally put in the two wheel drive state, and shifted to the four wheel drive state to prevent rear wheel lock and ensure vehicle stability when the brake is applied and the brake switch is turned on.
However, this system changes the drive system to the four wheel drive mode even when the brake pedal is depressed so slightly that the four wheel drive mode is not required. Therefore, this system causes uncomfortable feeling and deteriorates the durability of the 2WD-4WD change-over mechanism uselessly.
In general, a force acting in a tire to road contact surface is a vector sum of a driving force or braking force acting in a direction of wheel travel, and a side force acting laterally of the tire. The magnitude of a frictional force between a tire and a road surface is equal to a product of a weight of the vehicle body acting vertically on the wheel and a coefficient of friction between the tire and road surface. The above-mentioned vector sum cannot exceeds the magnitude of the frictional force. Therefore, when the driving or braking force is increased during rapid acceleration or hard braking, an upper limit of the side force is decreased, and the critical value of a cornering force related to the side force is decreased. As a result, the controllability and stability of the vehicle become worse. This undesired tendency is increased when the friction coefficient becomes low in rainy weather.